Continuous production of uniform graphite fibers

ABSTRACT

A rapid continuous process is provided for the conversion of a predominantly amorphous carbonaceous fibrous material containing at least 75 percent carbon by weight (preferably at least 90 percent carbon by weight) to a uniform fibrous material of predominantly graphitic carbon. The carbonaceous fibrous material is passed through a reducing flame which imparts a minimum fiber temperature of at least 1,900* C. while the fibrous material is under tension at least sufficient to prevent visible sagging. In a preferred embodiment of the invention, the reducing flame is generated by a fuel-oxidant mixture, e.g. an acetylene and oxygen mixture. Long lengths of graphite yarns having substantially uniform properties, e.g. graphitic composition, Young&#39;&#39;s modulus, and tenacity, may be produced through the use of the present process.

United States Patent 3,107,152 10/1963 Ford etal.

Dagobert E. Stuetz Westfield;

Leo R. Belohlav, Berkeley Heights, both of N.J.; Arthur M. Reader, Greenville, S.C.

Inventors Appl. No. 820,008

Filed Apr. 28, 1969 Patented Jan. 1 l, 1972 Assignee Celanese Corporation New York, N.Y.

Continuation-impart of application Ser. No. 614,811, Feb. 9, 1967, now abandoned. This application Apr. 28, 1969, Ser. No. 820,008

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CONTINUOUS PRODUCTION OF UNIFORM GRAPHITE FIBERS Primary Examiner-Edward J. Meros Attorneys-C. B. Barris, T. J. Morgan, Burns, Doane, Swecker and Mathis and K. E. Macklin ABSTRACT: A rapid continuous process is provided for the conversion of a predominantly amorphous carbonaceous fibrous material containing at least 75 percent carbon by weight (preferably at least 90 percent carbon by weight) to a uniform fibrous material of predominantly graphitic carbon. The carbonaceous fibrous material is passed through a reducing flame which imparts a minimum fiber temperature of at least 1,900 C. while the fibrous material is under tension at least sufficient to prevent visible sagging. ln a preferred embodiment of the invention, the reducing flame is generated by a fuel-oxidant mixture, e.g. an acetylene and oxygen mixture. Long lengths of graphite yarns having substantially uniform properties, e.g. graphitic composition, Youngs modulus, and tenacity, may be produced through the use of the present process.

CONTINUOUS PRODUCTION OF UNIFORM GRAPHITE FIBERS CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of our copending U.S. Ser. No. 614,81 1, filed Feb. 9, 1967, which is now abandoned.

BACKGROUND OF THE INVENTION In the search for high performance materials, considerable interest has been focused on graphite fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and which have a predominant X-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amphorous X-ray diffraction pattern. Graphite fibers generally have a much higher modulus and a higher tenacity than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.

Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and graphite fibers theoretically have among the best properties of any fiber, including boron, for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and most important, high modulus. Graphite is one of the very few known materials whose tensile strength increases with temperature. Uses for such graphite-reinforced composites include aerospace structural components, rocket motor casings, deep-submergence vessels and ablative materials for heat shields on reentry vehicles.

One of the major factors retarding the large scale use of graphite reinforced composites may be traced to the extreme costs commonly required for the production of high modulus graphite fibers suitable for use as reinforcement. Although the production of fibrous carbon by pyrolysis of hydrocarbon gases has been reported, this technique is generally not suitable for industrial applications requiring good quality control. Graphitization of fibrous organic precursors appears to be the only practical industrial route available to form graphite fibers.

Many of the prior art methods for producing graphite fibers involve long processing periods, high power requirements, and/or expensive and bulky heating apparatus, such as closed furnaces. The processing and equipment costs required to produce graphite fibers commonly dwarf the fiber raw material costs. Often the fiber is of inferior quality due to damage occurring in one or more steps used in its production. For example, amorphous carbon yarn can be converted to graphite yarn by furnace graphitization in a high temperature vacuum oven. In such an oven the hot zone material has commonly been a metal such as tungsten or tantalum. Besides being expensive and slow, this radiant heat method also may result in the deposition of foreign materials such as tungsten, tantalum and/or carbides of these metals on the fiber during the high temperature treatment.

Another prior art approach to graphitizing amorphous carbon fiber, called conductive graphitization, involves passing a yarn over electrically conductive contacts. For example, in one such method an amorphous carbon fiber is advanced over a pair of spaced electrically conductive rollers while passing an electric current through the advancing fiber to raise it to graphitization temperature. A controlled atmosphere of nitrogen, argon, or mixtures thereof, generally must be provided around the fiber while undergoing direct resistance heating. Additionally, equipment must commonly be provided to scrape off or otherwise remove residues on the contact rollers. Furthermore arcing tends to occur at the point of departure of the yarn resulting in local overheating and evaporation of portions of the yarn. The net result is a high incidence of inhomogeneity throughout a length of the resulting graphitized yarn. Many of the resulting graphitized fibers have low tensile strength and generally there is a wide distribution of individual break values. Apparently the combination of frictional contact of the yarn on the rollers during conductive graphitization and the arcing causes marked wear of the fiber and accordingly produces many flaws.

It is an object of the invention to provide a rapid, and continuous, process for the conversion of an essentially amorphous carbon fiber to a predominantly graphite carbon fiber.

It is an object of the invention to provide a process capable of producing a graphite fiber exhibiting relatively uniform properties.

It is another object of the invention to provide a graphitization process which does not require bulky and highly expensive equipment.

It is a further object of the invention to provide an appreciable length of graphite yarn characterized by all portions of the yarn having an X-ray pattern characteristic of graphite carbon, and substantially uniform Youngs modulus and tenacity values.

SUMMARY OF THE INVENTION under a tension at least sufficient to prevent visible sagging.

Through the use of the present process, one may produce a package containing more than 10 grams of uniform graphite yarn characterized by all portions of the yarn of the package having an X-ray diffraction pattern characteristic of graphite carbon, and an average deviation from both the average Youngs modulus value and the average tenacity value of less than fi percent.

DETAILED DESCRIPTION OF THE INVENTION A carbonaceous fibrous material suitable for graphitization according to the method of this invention has (I) at least 75 percent of its fiber weight attributable to carbon, and preferably at least 90 percent of its fiber weight attributable to carbon, (2) a relatively amorphous X-ray diffraction pattern, and (3) structural integrity for at least two seconds at a fiber temperature of I,900 C.

The carbonaceous fibrous material selected for use in the present process may be formed by a variety of techniques which are known in the art. Such carbonaceous fibrous materials are generally formed by first stabilizing an organic polymeric fibrous material and subsequently pyrolyzing or carbonizing the same under conditions whereby its fibrous configuration is maintained. Organic polymeric fibrous materials of varied chemical constitution can be selected as precursors from which the carbonaceous fibers utilized in the process are derived including such diverse types as cellulosics, e.g. regenerated cellulose, cotton, and cellulose acetate; nitrogenous heterocyclics such as polybenzimidazoles; and acrylics consisting primarily of recurring acrylonitrile units. The preferred acrylic is an acrylonitrile homopolymer; however, acrylonitrile copolymers may be selected which contain at least about mol percent of acrylonitrile units and up to about 15 mol percent of one or more monovinyl units copolymerized therewith.

Amorphous carbon yarns suitable for use in the process are commercially available. Organic polymeric fibrous materials of a cellulosic origin are commonly stabilized prior to carbonization by then'nal treatment in a nonoxidizing atmosphere, while fibrous materials of a nitrogenous heterocyclic or acrylic origin are commonly initially stabilized by heating in the presence of oxygen. Illustrative, carbonization procedures for organic fibrous precursors are disclosed, for example, in US. Pat. Nos. 3,053,775 to Abbott, 3,235,323 to Peters, 3,285,696 to Tsunoda, 3,313,596 to Hogg et al., and in French Pat. No. 1,430,803.

The carbonaceous fibrous materials which graphitized in accordance with the present invention are preferably in yarn form. Appreciable lengths of continuous multifilament yarns are graphitized in a particularly preferred embodiment of the invention. Staple yarns may also be selected, however, these generally give correspondingly lower tensile properties than do the continuous filament yarns. ()ther fibrous assemblages, such as carbonaceous fabrics, may likewise be treated in accordance with the present invention as will be apparent to those skilled in the art.

When a carbonaceous yarn is selected as the starting material it may optionally be provided with a twist which improves its handling characteristics. For example, a twist of about 0.1 to l t.p.i., and preferably about 0.1 to 0.7 t.p.i. may be conveniently utilized.

A highly preferred reducing flame for use in the process is that resulting from an acetylene and oxygen mixture. With this reducing flame, the graphitizing step can be conducted in an open atmosphere. A further advantage of the acetylene-oxygen reducing flame is that is has a fairly constant high temperature which is independent, within limits, of the fuel-oxidant ratio. A carbon monoxide-oxygen flame also yields good results in an open atmosphere, although it is, of course, essential to provide adequate safety provisions for the operator under such conditions. Hydrocarbon fuels, such as propane and butane, may be selected but the process does not proceed as smoothly as with acetylene or with carbon monoxide. However, in the presence of an inert blanketing gas, e.g. nitrogen, or argon, comparable stability may be achieved with hydrocarbon fuels.

Molecular oxygen can be replaced in the fuel-oxydant mixture by a gaseous oxidant such as nitrous oxide although generally it is not advantageous to do so because of the convenience and ready availability of oxygen. Fuel-oxidant combinations can also be employed to produce the reducing flame which do not contain a hydrocarbon fuel, such as a carbon monoxide-hydrogen mixture and a hydrogen-chlorine mixture. Nonconventional flame sources, such as augmented flames (cf. B. Karlovitz, International Science of Technology, June 1962, pp. 36-41); recombination flames, such as the atomic hydrogen torch (cf. 1. Langmuir, Industrial and Engineering Chemistry, June 1927, pp. 667-674); plasma torches; and the like; can also be employed to provide high temperatures. The temperature should not be so high, however, as to destroy the fibrous configuration.

The fuel to oxidant ratio generally is a significant parameter in the present process. The graphitizing treatment is best carried out in a luminous flame obtained by keeping the amount of oxygen in the fuel mixture below the stoichiometric amount which is required to burn the fuel completely to carbon dioxide. Oxidation reactions within the flame are essentially limited to the combustible gas mixture, and the fibrous material traveling through the flame is exposed to an essentially reducing environment. The fibrous material can be destroyed by oxidation if the oxidant-fuel ratio is too high. The luminosity of the flame is believed to be caused by ionized carbon fragments in the flame resulting from incomplete combustion of the fuel. Less pyrolytic carbon will be deposited at higher oxygen to fuel ratios than at lower ratios. For certain applications a deposit of pyrolytic graphite is desirable since it increases the high temperature stability of the resulting graphitic product. For other applications such a deposit is undesirable, e.g., where high adhesion to a matrix is desired. Hence, the flexibility of processing conditions allows for the production of a variety of graphite product types. If desired, a surface protective layer of pyrolytic graphite alternatively can be formed in a separate step in which the fibrous material is heated to a high temperature in a controlled environment containing hydrocarbon vapors.

ln the context of this specification, temperatures in the flame zone refer to the temperature of the fibrous material as measured by an infrared radiation thermometer and not to a theoretical reducing flame temperature under adiabatic conditions, i.e. without withdrawal of heat by immersing a body into the reducing flame. The temperature of the fibrous material in the flame is generally significantly lower than the theoretical reducing flame temperature.

For example the theoretical flame temperature of an oxygen-acetylene reducing flame is about 3,l00 C. An upper limit of about 2,500 C. for the fiber temperature of a yarn undergoing treatment is generally sufficient and safe.

The amorphous carbonaceous fibrous material is passed through the reducing flame at a fast enough rate to avoid breaking. As the temperature of the reducing flame is increased, the minimum rate at which breakage is avoided also increases. This minimum speed can be determined for any given combination of amorphous carbonaceous fibrous material and specific reducing flame. The longer the residence time, the greater the extent of graphitization. Thermal degradation with graphite formation occurs within the reducing flame to the substantial exclusion of oxidative degradation. Optimum conditions are reached at the point where the loss of the fiber mass is lowest and the conversion of the remainder of the fibrous material to graphitic carbon is the highest. The two effects can be balanced favorably by adjusting the residence time and the fiber temperature. Subject to the nature of the reducing flame and other factors, residence times of about 2 to 24 seconds, and preferably about 6 to 17 seconds are generally suitable. An exemplary set of optimum conditions is a yarn temperature of 2,300 C. with a residence time of about 15 seconds.

The necessary apparatus for graphitization according to the present process is simple and should be so arranged that the fibrous material is exposed to a minimum of frictional contacts. A convenient arrangement is to continuously feed an amorphous carbonaceous yarn from a rotating-reciprocating bobbin through the reducing flame to an identically functioning takeup mechanism. Starting at the correct reciprocating position on the bobbin, the yarn is unwound without traverse movement and analogously rewound at the takeup side. Hence, random yarn motions are minimized. Further positioning of the yarn in the reducing flame can be accomplished with minimal action by two cylindrically shaped guides located before and after the reducing flame source. Feed and takeup bobbins can be driven by, for example solid-state controlled DC motors with r.p.m. generator/indicators. When an inert atmosphere is desired, a cruciform glass vessel fitted with cooling plates and a passage opening for the yarn can be employed. Before entering the burner module, the yarn may be put under constant tension as, for example, by passing it over a rubber-capped electromagnetic clutch and a skewed roll. The latter separates individual yam loops around the clutch and prevents abrasion by yarn to yarn contact.

The geometry of the burner is also a factor in maximizing the effectiveness of the graphitization of the present invention. Two impinging flames originating from two standard conical tips significantly raise the temperature of the fibrous material passing therethrough. A particularly preferred embodiment of this technique is the impingement of the two flames on the tips of their inner cones at an angle of 45. However, the addition of more than two orifices does not have a beneficial effect. A series of burners such as to form a continuous reducing flame zone of increased lateral dimension may permit increased processing speeds or the processing of larger fibrous assemblages. Since residence time in the reducing flame is a major parameter, processing speed is significantly related to the length of the flame zone. Surrounding the conical tip with a cylindrical or globular reflector, constructed from polished stainless steel sheets, for example, also significantly raises the yarn temperature.

The volume flow of the fuel and oxidant through the burner should be as high as possible, consistent with good reducing flame stability, in order to maximize the moduli of the fibers.

Tension during reducing flame treatment is recommended for the achievement of optimum properties as it prevents the tendency of the fibrous material to shrink. Shrinkage usually leads to relaxation of ordered structures and, thereby, causes a lowering of physical properties. Preservation of orientation and/or an increase in orientation, depending upon the magnitude of tension applied, increases both the Youngs modulus and the tensile strength. The tension applied should be at least sufiicient to avoid visible sagging. Beyond the optimum tension the fibrous material may be damaged by still higher ten- SlOi'iS.

Flamcproofing compounds may optically be applied to the carbonaceous fibrous material prior to its passage through the reducing flame. Such flame proofing or protective agents are not essential in order to effectively carry out the process of the invention. For instance, treatment of an amorphous carbonaceous yarn containing at least 75 percent carbon by weight (preferably 90 percent carbon) prior to passage through the reducing flame with an aqueous boric acid solution (e. g. about 5 to 20 percent by weight) may strengthen the yarn against the rigorous graphitization conditions. Thus higher reducing flame temperatures and/or longer residence times can sometimes be better tolerated than with nontreated yarns and the properties of the resulting graphite fibrous material enhanced. Other materials which can be optionally employed include silicone oil (DOW 700), antimony salts, and the common bromineand chlorine-containing flame-proofing compounds.

Graphite fibers produced according to the present process have a relatively higher average tensile strength and a much narrower distribution of individual break values than are generally obtainable by conductive graphitization. Particularly uniform fibers have been obtained when the present process is carried out with a carbonaceous fibrous material derived from a polybenzimidazole. Although we do not wish to be limited by the theory of this improvement, it appears to be due to two factors: (1) the flame healing of flaws by ionized carbon fragments in a luminous reducing flame, and (2) the reduced mechanical wear of the fibrous material in the reducing flame as contrasted to the frictional contact of the rollers during conductive graphitization.

The following examples are given as specific illustrations of the invention. It should be understood, however, that the invention is not limited to the specific embodiments which follow.

The data in table I illustrate the results obtainable in an embodiment of the invention employing an acetylene-oxygen flame source as the reducing flame. The carbonaceous yam employed was derived from a cellulosic yarn, possessed a slight twist, was sold commercially under the designation VYB-70l0 (Union Carbide), had a total denier of 680, and had the physical properties identified in table I. The yarn exhibited an amorphous X-ray diffraction pattern, and in excess of 90 percent of its fiber weight was attributable to carbon. In the manner described above the yarn was passed in the open atmosphere through an acetylene-oxygen reducing flame in which the volume ratio of the gases was 1:1. The total flow rate was 1,500 ml./min. and a National Blow Torch, Tip AO3 was employed. The acetylene-oxygen reducing flame imparted a fiber temperature of about 2,200 to 2,400 C. to the yarn. The tension applied to the yarn was 90 grams or 0.13 grams denier. No visible sagging of the yarn was apparent. The following table l summarizes the properties of the resulting graphite yarn (average of five single filament breaks) as a function of the residence time of the yarn in the reducing flame.

TABLE I Graphite Youngs crystal- Residence time in Tenacity modulus linity reduclng flame Tension (grams/ (grams/ (X-ray (seconds) (grams) denier) denier) analysis) VYB-70-l/O Carbon yarn:

3.5 151 NO. 6.7 630 Yes. 4 80 15.8, H680 wYes.v

6 5.4 486 Yes. 90 3.6 468 Yes. 90 3.3 474 Yes. 90 1.4 310 Yes.

The data in table ll illustrate the results obtainable in an em I v bodiment of the invention employing a carbonaceous yarn derived from a polybenzimidazole yarn to form a graphite yarn. The exemplary polybenzimidazole was poly[2,2"(mphenylene)-5,5'-bibenzimidazole], commonly referred to as PB]. The polybenzimidazole precursor can be conveniently prepared by the method of example ll of US. Pat. No. 2,895,948 to Brinker et al., or according to the teachings of US. Pat. No. 3,174,947 to Marvel et al. A yarn can be prepared from the exemplary polybenzimidazole by dry spinning from dimethylacetamide, for example, in a manner known to the art. The resulting PBl yarn is preoxidized by heating in air for about 6 minutes at 450 C. and subsequently carbonized at 800 C. in an inert atmosphere for a residence time of 8 minutes. The carbonized PBl yarn contained in excess of 90 percent carbon by weight, possessed a slight twist of about 0.5 t.p.i., had a total denier of 1,450, and exhibited an amorphous X-ray difiraction pattern. The same reducing flame source and processing conditions were employed as described in connection with the runs reported in table 1. Highly uniform physical properties were exhibited by the resulting graphite yarn.

I V The run sreported table lIl illustratethe use of a different reducing flame source, i.e. a mixture of oxygen and propane, to produce graphite yarn. The same carbonaceous yarn as used in the runs reported in table I was employed. The yarn was passed through an oxygen-propane reducing flame in which the volume ratio of the gases was varied as indicated in table ill, and a fiber temperature of approximately 2,000 C. was imparted to the yarn. Sufficient tension was maintained upon the yarn to prevent visible sagging. The apparatus included a cruciform glass vessel with provision for maintaining a nitrogen atmosphere therein, and was fitted with cooling plates and passage openings for the fiber. A surface-mix propane-oxygen burner was mounted within the vessel. The data show the properties of the resulting graphite yarn (average of 10 single filament breaks) as a function of the oxygen-propane ratio and the yarn residence time in the reducing flame.

Carbon yarn immersed in a boric acid solution at room temperature (5 per cent by Weight in distilled water) and air dried prior to exposure to the reducing flame.

NFt that an increase in the oxygen rzTioETen at shortened residence times leads to a reduction in tenacity. By boric acid treatment of the yarn prior to flame treatment, the detrimental effect of the higher oxygen supply in the reducing flame is compensated.

The runs reported in table IV illustrate the results obtainable in an embodiment of the invention employing a carbonaceous yarn derived from a 760 continuous filament acrylonitrile homopolymer yarn to form a graphite yarn. The acrylonitrile homopolymer yarn had a twist of about 0.5 t.p.i. The carbonaceous yarn derived from an acrylonitrile homopolymer yarn exhibited the physical properties identified in table IV, and was formed by preoxidizing and carbonizing the precursor as described below. The acrylonitrile homopolymer was preoxidized in air in accordance with the teachings of US. Ser. No. 749,957, filed Aug. 5, 1968 of Dagobert E. Stuetz, which is assigned to the same assignee as the instant invention, and exhibited a bound oxygen content of about 8 percent by weight as determined by the Unterzaucher analysis and a carbon content of about 62 percent by weight. The preoxidized yarn prior to carbonization was nonbuming when subjected to an ordinary match flame, but was incapable of withstanding the reducing flame utilized in the present process which imparts a highly elevated fiber temperature of at least l,900 C. The preoxidized yarn was carbonized to form a carbonaceous yarn exhibiting a total denier of approximately 1,000, an amorphous X-ray diffraction pattern, and a carbon content in excess of 90 percent by weight, by heating at 1,050" C. in a constant temperature muffle furnace containing a nitrogen atmosphere for about 2 minutes. The same reducing flamesource and processing conditions were employed as described in connection with the runs reported in table I. No visible sagging of the yarn was observed.

Prior to flame graphitization the carbonized yarn was continuously passed through a 10 percent by weight solution of tris (2,3 dibromopropyl) phosphate in trlchloroethylene at room temperature. This protect ve agent had essentially no effect as indicated in the data.

Tables I, ll, Ill, and IV illustrate improved fiber properties achievable by the method in this invention.

By the method of this invention one can form even substantial packages of uniform graphite yam, i.e. containing more than 10 grams, in which all portions of the yarn have l an X- ray difiraction pattern characteristic of graphite, and (2) an average deviation from both the average Youngs modulus value and the average tenacity value of less than fi percent. Ten grams of the graphite yarn of this invention is of the order of magnitude of 100 feet in length. Such a package of uniform graphite yarn is particularly suited in those applications where property uniformity and reliability are necessary.

Although the invention has been described with preferred embodiments it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and scope of the claims appended hereto.

We claim:

1. A continuous process for producing a predominantly graphitic fibrous material comprising continuously passing a carbonaceous fibrous material having:

a. at least 90 percent of its weight attributable to carbon,

b. a relatively amorphous X-ray diffraction pattern, and

c. structural integrity for at least 2 seconds at a fiber temperature of l,900 C., through the luminous portion of a reducing flame generated by a fuel-oxidant mixture imparting to said fibrous material a minimum temperature of at least l,900 C. in which the ratio of oxidant to fuel is less than the amount required to completely oxidize the fuel for a residence time of about 2 to 24 seconds at a speed sufficient to avoid breaking while said fibrous material is under a tension at least sufficient to prevent visible sagging.

2. A continuous process according to claim 1 wherein said oxidant is oxygen.

3. A continuous process according to claim 1 wherein said fuel is acetylene.

4. A continuous process according to claim 2 wherein said fuel is propane.

5. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from a cellulosic fibrous material.

6. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from a polybenzimidazole fibrous material.

7. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from an acrylic fibrous material consisting primarily of recurring acrylonitrile units.

8. A continuous process according to claim 7 wherein said carbonaceous fibrous material has been derived from an acrylonitrile homopolymer.

9. A continuous process according to claim 1 wherein the residence time of said carbonaceous fibrous material in said reducing flame is from about 6 to 17 seconds.

10. A continuous process according to claim 4 wherein an inert atmosphere is provided around said reducing flame.

11. A continuous process according to claim 1 wherein said carbonaceous fibrous material is a yarn.

12. A continuous process for producing a predominantly graphitic fibrous material comprising continuously passing a carbonaceous fibrous material having:

a. at least percent of its weight attributable to carbon,

b. a relatively amorphous X-ray diffraction pattern, and

c. structural integrity for at least two seconds at a fiber temperature of l,900 C., through the luminous portion of a reducing flame generated by an acetylene-oxygen mixture imparting to said fibrous material a minimum temperature of at least l,900 C. in which the ratio of oxygen to acetylene is less than the stoichiometric amount required to completely oxidize the acetylene for a residence time of about 2 to 24 seconds while said fibrous material is under a tension at least sufficient to prevent visible sagging.

13. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from a cellulosic fibrous material.

14. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from a polybenzimidazole fibrous material.

15. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from an acrylic fibrous material consisting primarily of recurring acrylonitrile units.

16. A continuous process according to claim 15 wherein said carbonaceous fibrous material has been derived from an acrylonitrile homopolymer.

17. A continuous process according to claim 12 wherein the residence time of said carbonaceous fibrous material in said reducing flame is about 6 to 1 7 seconds.

18. A continuous process according to claim 12 wherein said carbonaceous fibrous material is a yarn. 

2. A continuous process according to claim 1 wherein said oxidant is oxygen.
 3. A continuous process according to claim 1 wherein said fuel is acetylene.
 4. A continuous process according to claim 2 wherein said fuel is propane.
 5. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from a cellulosic fibrous material.
 6. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from a polybenzimidazole fibrous material.
 7. A continuous process according to claim 1 wherein said carbonaceous fibrous material has been derived from an acrylic fibrous material consisting primarily of recurring acrylonitrile units.
 8. A continuous process according to claim 7 wherein said carbonaceous fibrous material has been derived from an acrylonitrile homopolymer.
 9. A continuous process according to claim 1 wherein the residence time of said carbonaceous fibrous material in said reducing flame is from about 6 to 17 seconds.
 10. A continuous process according to claim 4 wherein an inert atmosphere is provided around said reducing flame.
 11. A continuous process according to claim 1 wherein said carbonaceous fibrous material is a yarn.
 12. A continuous process for producing a predominantly graphitic fibrous material comprising continuously passing a carbonaceous fibrous material having: a. at least 90 percent of its weight attributable to carbon, b. a relatively amorphous X-ray diffraction pattern, and c. structural integrity for at least two seconds at a fiber temperature of 1,900* C., through the luminous portion of a reducing flame generated by an acetylene-oxygen mixture imparting to said fibrous material a minimum temperature of at least 1,900* C. in which the ratio of oxygen to acetylene is less than the stoichiometric amount required to completely oxidize the acetylene for a residence time of about 2 to 24 seconds while said fibrous material is under a tension at least sufficient to prevent visible sagging.
 13. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from a cellulosic fibrous material.
 14. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from a polybenzimidazole fibrous material.
 15. A continuous process according to claim 12 wherein said carbonaceous fibrous material has been derived from an acrylic fibrous material consisting primarily of recurring acrylonitrile units.
 16. A continuous process according to claim 15 wherein said carbonaceous fibrous material has been derived from an acrylonitrile homopolymer.
 17. A continuous process according to claim 12 wherein the residence time of said carbonaceous fibrous material in said reducing flame is about 6 to 17 seconds.
 18. A continuous process according to claim 12 wherein said carbonaceous fibrous material is a yarn. 